Process for treatment of molten stainless steel



Mi... June a, 1950 PROCESS FOR TREATMENT OF MOLTEN- STAINLESS STEELHarry Tancwn, Baltimore, Md., assig'nor, by

mesne assignments, to Armco vSteel Corporation, a corporationof Ohio NoDrawing. Application July 11, 1945, Serial No. 604,542

'IClaims.

This invention is a companion to that of my copending application,Serial No. 604,541, filed July 11, 1945, and entitled Stainless steelprocess and product and relatesto stainless steel, more especially to anart of producing castings of stainless steel and to flux compositionsfor use against scums of teeming and casting the steel.

One of the objects of my invention is the eflicient and economicalproduction of cast stainless steel products, such as ingots, which aresubstantially free of scum occlusions ordinarily resulting fromstainless steel teeming and casting operations.

Another object of my invention is the production of stainless steelcastings of the character indicated in a simple, practical andthoroughly reliable manner from any of a wide variety of stainlesssteels utilizing known and tried casting apparatus.

A further object of my invention is the provi-- sion from readilyavailable material or materials of an efficacious addition agent forfiuxing stainless steel teeming scums from the steel.

Other objects in part will be obvious and in part will be pointed outhereinafter.

The invention accordingly consists in the combination of elements,composition of ingredients and mixture of materials, in the operationalsteps, and in the relation of each of the same to one or more of theothers as described herein, the scope of the application of which isindicated in the following claims. I

As conducive to a clearer understanding of certain features of myinvention it may be noted at this point that the stainless steels aredefined as low-carbon steels which comprise to 35% chromium, with orwithout nickel, and with or without supplemental additions of columbium,titanium, aluminum, molybdenum, tungsten, vanadium, cobalt, copper,manganese, silicon, sulphur, and the like, for special purposes, and aremainder which is substantially all iron. The carbon content may rangefrom 0.03% to 0.20%, or more where desired.

In the production of stainless steel castings a serious problem ariseswhich has to do with the prevention or elimination of scums which eventually prove to be hurtful to the cast metal. These scums develop inpouring the steel as to ingot molds and usually are rich in oxides bothof iron and chromium or on occasions also include in quantity oxides ofspecial purpose constituents of the steel. Likewise, nitrides of.certain elements of the alloy steel usually contribute as a further scumcomponent along with the oxides. At times, the scum assumes a refractoryor semi-refractory quality which is thick, viscous,

frothy, gummy, or even semi-solid and does not readily melt in contactwith the molten stainless steel in the mold.

As a matter of usual occurrence these refractory scums are driven intothe newly cast stainless steel by the force of the pouring stream or areturned under at the faces of the mold as the metal rises or areotherwise forced to inclusion in the-metal during the teeming andcasting operations. The scums are sluggish and in part remain entrapped,to the result that the castings after solidification require aconsiderable amount of surface conditioning such'as grinding to removenon-metallics and metal of inferiorquality, or at times the castings arerejected due to inferior quality, which in any event is objectionableand creates a need for more efiicient stainless steel casting practices.In some cases scum inclusions ofier an obstacle to successful forgingand rolling as by causing the metal to crack and chip at the-edges, ormake the metal unsuitable for those uses where high quality or strengthis important. The cast stainless steels which contain segregations ofscum therefore are suscep-:

tible to a host of metallurgical failures.. They are not successfullyfinished as for example to uniform luster and brightness and are undulysubject to corrosion in localized surface areas.

Some of the stainless steels, as when poured in contact with theatmosphere,'are more susceptible than others to the formation ofdeleterious aluminum, titanium, columbium, or the like,

whether the chromium content be relatively large or small. Y.

The fluidiilcation or alleviation of the deleterious scums of teemingand casting stainless steels heretofore has been attempted with variousfluxing agents only with quite limited success and with markedinefiectiveness in theinstances of;

igh chromium steels and steels including any such elements as titanium,columbium, aluminum, or the like, which produce especially ."efractoryscums. e

An object of, my invention accordingly is" the provision of a simple,direct and practical method of teeming and casting stainless steel,which method is highly efn'cient in preventing or elim- I inatingdeleterious scums of teeming with respect 'ing into ingots.-

to any of a wide variety of stainless steels, even the high temperaturestainless steels and those steels including one or more such elements astitanium, columbium, and aluminum.

Referring now more particularly to the practice of my invention I findthat by introducing to molten stainless steel alkali metal fluorides ofone or more of the three relatively light alkali In accordance with myinvention I add to the molten stainless steel, as in the production ofingots of the steel, a scum-fluidifying agent comprising inpredominating amount or preferably consisting substantially all of oneor more of the above noted alkali metal fluorides, for example to aladle of the steel before lip pouring into a mold, or to the-steel inteeming the same to the mold, or at the mold as by sifting the alkalimetal salt inside the mold over the rising steel.

The fluoride of lithium, sodium or potassium, either alone or togetherwith one or more of each other, are highly effective and reliablestainless steel scum combattingmaterials even in the casting of hightemperature stainless steels such as those comprising 20% or morechromium, and stainless steels including at least one such element astitanium, aluminum, columbium tor some other element or elements havinga tendency to produce a highly refractory nonmetallic scum in and on-themetal during the casting operations whether the chromium content of thesteel be high or low; I find advantage at times in using a proportion oflithium fluoride, which is a very powerful stainless steel scum fluxingagent, in combination with one or more of sodium fluoride and potassiumfluoride which are relatively inexpensive yet quite powerful fluxingmaterials.

Through treating the molten stainless steel with one or more of thelightalkali metal fluorides the resulting stainless steel ingots or othercast products which Iachieve are of highly desirable qualitysubstantially free of teeming scum inclusions. Likewise, the stainlesssteel castings have a much superior surface and are remarkably devoid ofscabs, cold-shuts, wrinkles, rolls, splash marks, and like defects. Thestainless steel ingots which result from my pouring and casting methodare satisfactory for reducti0n-as by rolling with a minimum ofpreliminary conditioning of the surface thereof. The ingots give a highhot working yield with either splitting checking along the forged orrolled edges, ordinarily caused by refractory scum or other surfaceflaws.

As illustrative of the practice of my invention I provide, in a suitableelectro-metallurgi-cal melting furnace, a bath of 18-8 chromium-nickelstainless steel of the 2% titanium grade for cast- I tap the metal fromthe furnace into a ladle preparatory to teeming the same into ingotmolds. I preparing the molds' themselves, I put inside on'the bottom ofeach a.

' chromium and aluminum.

small quantity, illustratively about 2 to 4 ounces, of a scum-combattingagent consisting substantially all of lithium fluoride and a smalleramount of sodium fluoride, the agent preferably being enclosed in smallpaper bags or the like. Thereafter, I teem the stainless steel from theladle into molds and over the alkali metal salts in the same.

The lithium fluoride and sodium fluoride in the ingot molds thoroughlymix with the molten steel during the teeming and casting operations andset up a highly effective fiuxing action in preventing and eliminatingthick s-cums in the mold. If desired, additions of fluxing agent aremade to the mold as by sprinkling onto the cast metal as the teemingcontinues or by mixing into the cast molten metal after teeming, tosupplement or serve instead of previous additions of the salts to themold. I let the scum-fluxed ingots solidify and cool, and thereafterstrip of! the molds. The resulting chromium-nlckelaluminum steel ingotsare of good surface quality and are substantially free of scuminclusions such as those including refractory oxides of In the strippedcondition, the ingots are ready for storage or sale, or for example areeasily made ready for forging or rolling and subsequently are formedinto reduction products.

Thus it will be seen that there has been provided in this invention amethod and de-scumming agent for the production of cast stainless steelproducts, and the stainless steel ingots or other cast productsthemselves, in which the various objects hereinbefore set forth togetherwith many thoroughly practical advantages are successfully achieved.- Itwill be seen that the method includes the use of a powerful descummingagent, is simple, effective and suitable for the production of cleancastings from any of a wide variety of stainless steel, and isindustrially practicable with known and tried casting apparatus.

As many possible embodiments may be made of my invention and as manychanges may be made in the embodiments hereinbefore set forth, it is tobe understood that all matter described herein is to be interpreted asillustrative and not as a limitation.

I claim as my invention:

1. In the production in a mold of cast stainless steel productssubstantially devoid of scabs,

cold shuts, wrinkles, rolls and splash marks, the

art which includes contacting molten steel during the teeming thereof,and the scum formed by contact of the molten steel with the atmosphere,witha teeming scum flux essentially consisting of atleast two of thegroup consisting of lithium fluoride, sodium fluoride and potassiumfluoride.

2.- In the production in a mold of cast stainless steel productssubstantially devoid of scabs, cold shuts, wrinkles, rolls and splashmarks, the art which includes adding to a stream of molten steel duringthe teeming thereof, and the scumformed by contact of the molten steelwith the atmosphere, a teeming scum flux essentially consisting oflithium fluoride and at least one of the group consisting of sodiumfluoride and potassium marks in which the steel contains one or more oftitanium, columbium or aluminum, the are which includes adding to steelduring the teeming therestainless steel cast products containing atleastv 20% chromium and substantially devoid of scabs, cold shuts,wrinkles, rolls and splash marks, the art which includes adding to themold a teeming scum flux essentially consisting of at least one of thegroup consisting of sodium fluoride and potassium fluoride. I

' 5. In the production of stainless steel ingots substantially devoid ofscabs, cold shuts, wrinkles, rolls and splash marks, the art whichincludes placing in an ingot mold a teeming scum flux essentiallyconsisting of lithium fluoride and sodium fluoride, and teeming themetal therein.

6. In the production of stainless steel ingots substantially devoid ofscabs, cold shuts, wrinkles, rolls and splash marks, the art whichincludes teeming molten steel into an ingot mold, and during the teemingsprinkling on the surface of the metal rising in the mold a teeming scumflux essentially consisting of lithium fluoride and sodium fluoride.

7. In the production of stainless steel ingots substantially devoid ofscabs, cold shuts, wrinkles, rolls and splash marks, the art whichincludes mixing with metal in an ingot mold a teeming scum fluxessentially consisting of lithium fluo- 80 ride and at least one of thegroup consisting of sodium fluoride and potassium fluoride.

HARRY TANCZYN,

REFERENCES CITED The following references are of record in ,the tile 01this patent:

UNITED STATES PATENTS Number Name Date 156,009 Atwood Oct. 20, 18741,672,446 Cape June 5, 1928 1,748,217 Grossman Feb. 25, 1930 2,265,985Allen Dee. 16,-1941 FOREIGN PATENTS I Number Country Date 110,022Austria Mar. 28, 1940 423,489 Great Britain Feb. 1, 1935 476,115 GreatBritain Dec. 2, 1937 509,442 Great Britain July 17, 1939 OTHERREFERENCES Handbook of Chemistry and Physics, 28th edition, pages 402 to404. Published in 1944 by the Chemical Rubber Publishing 00., Cleveland,Ohio.

Refining Metals Electrically, page 140. Edited by Barton. Published in1926 by the Penton Publishing 00., Cleveland. Ohio.

1. IN THE PRODUCTION IN A MOLD OF CAST STAINLESS STEEL PRODUCTS SUBSTANTIALLY DEVOID OF SCABS, COLD SHUTS, WRINKLES, ROLLS AND SPLASH MARKS, THE ART WHICH INCLUDES CONTACTING MOLTEN STEEL DURING THE TEEMING THEREOF, AND THE SCUM FORMED BY CONTACT OF THE MOLTEN STEEL WITH THE ATMOSPHERE, WITH A TEAMING SCUM FLUX ESSENTIALLY CONSISTING OF AT LEAST TWO OF THE GROUP CONSISTING OF LITHIUM FLUORIDE, SODIUM FLUORIDE AND POTASSIUM, FLUORIDE. 